The present invention relates to oligonucleotides that can be used to affect the activity of target RNAs.
The first generation of such oligonucleotides were antisense oligonucleotides that were intended to affect the activity of target mRNAs. One reason for interest in such oligonucleotides is the potential for exquisite and predictable specificity that can be achieved because of specific base pairing. In other words, it is in theory very simple to design an oligonucleotide that is highly specific for a given nucleic acid, such as an mRNA.
However, it has turned out simple base pairing is often not enough to achieve regulation of a given target mRNA, i.e. an oligonucleotide complementary to a given target mRNA does not necessarily affect the activity of the target mRNA. If the oligonucleotide targets the open reading frame of an mRNA, it may e.g. be that the translational apparatus simply displaces the oligonucleotide during translation. Therefore, means where developed that would improve the regulatory activity of the oligonucleotide.
E.g. oligonucleotides that can activate RNase H cleavage of the target mRNA were developed. One potential disadvantage of such oligonucleotides is that they may mediate cleavage of other RNAs than the intended target mRNA, i.e. giving rise to off-target effects. Still, such oligonucleotides acting through RNase H cleavage are in clinical trials for treatment of various diseases.
Recently, research has shown that eukaryotic cells, including mammalian cells, comprise a complex gene regulatory system (herein also termed RNAi machinery) that uses RNA as specificity determinants. This system can be triggered by so called siRNAs that may be introduced into a cell of interest to regulate the activity of a target mRNA. Currently, massive efforts go into triggering the RNAi machinery with siRNAs for specific regulation of target RNAs, in particular target mRNAs. This approach is widely regarded as having great promise for the development of new therapeutics. As will also be outlined below, a major advantage of this approach is that specificity of the siRNA lies in the degree of complementarity between the guide strand of the siRNA and the target RNA, i.e. target specificity can be controlled. However, it has turned out that siRNAs may be less specific than initially thought. Initially, it was believed that only target RNAs that harboured stretches of complete complementarity to the guide strand of the siRNA would be affected, i.e. targeted by the RNAi machinery. New research indicates that siRNAs indeed do result in significant off-target effects, i.e. regulation of non-intended targets. It is now believed that these off-targets stem from the siRNAs, or rather the guide strand of the siRNAs, acting as microRNAs.
MicroRNAs are a class of endogenous RNA molecules that has recently been discovered and that, as siRNA, function via the RNAi machinery. Currently, about 500 human microRNAs have been discovered and the number is rapidly increasing. It is now believed that more than one third of all human genes may be regulated by microRNAs. Therefore, microRNAs themselves may be used to regulate the activity of target RNAs, and consequently e.g. be used as therapeutics.
However, microRNAs generally act at more than one target RNA, i.e. they are promiscuous. Thus, introduction of a microRNA into the cell or regulating the level of a microRNA will affect the activity of more than one target mRNA and consequently often give rise to undesired off-target effects or unintended effects.
A recent approach has been put forward, wherein the activity of a target RNA is regulated by inhibiting the activity of a microRNA. The microRNA can be inhibited using complementary oligonucleotides that have been termed antimirs and antagomirs. Since the microRNA is itself promiscuous, the antimir or antagomir will affect the activity of more than one target RNA.